Resistant Crater Ejecta

The ejecta of this unnamed crater in Amazonis Planitia is more resistant than the material around it. Scientists think these craters form when the force of an impact throws an apron of ejecta around a new crater. When the debris comes to rest, it contains mostly rock, mixed perhaps with some water from the subsurface. Extending outward, the apron armors the ground against erosion.

As years pass, scientists argue, the topmost layers of the surrounding plains erode due to winds or climate change or other factors. This lowers the ground around the crater, except where the ejecta covered it.